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astroengine writes: 'How cosmic dust is created has been a mystery for some time. Although the textbooks tell us that the dusty stuff that builds the planets — and, ultimately, the complex chemistry that forms life (we are, after all, made of 'star stuff') — comes from supernova explosions, astronomers have been puzzled as to how delicate grains of dust condense from stellar material and how they can possibly survive the violent shock waves of the cataclysmic booms. But now, with the help of a powerful ground-based telescope, astronomers have not only watched one of these supernova 'dust factories' in action, they've also discovered how the grains can withstand the violent supernova shock. "When the star explodes, the shockwave hits the dense gas cloud like a brick wall," said lead author Christa Gall, of Aarhus University, Denmark. "It is all in gas form and incredibly hot, but when the eruption hits the 'wall' the gas gets compressed and cools down to about 2,000 degrees. At this temperature and density elements can nucleate and form solid particles. We measured dust grains as large as around one micron (a thousandth of a millimeter), which is large for cosmic dust grains. They are so large that they can survive their onward journey out into the galaxy (PDF)." The surprising size of the measured dust particles means they can better survive the supernova's shockwave. This research has been published in the journal Nature.

I wish the author had elaborated a bit on this. I know that we're far away from the "ideal gas" regime here, and that things get independently wonky when you're dealing with supersonic flow, but "cooling down as it gets compressed" is so counter-intuitive that they should throw us at least a few lines of explanation.

The only thing I can think is it works like a refrigerator.The shock wave compress the the dust, which raises its temperature, so it radiates heat (a lot of heat: P ~ T^4).When the shock wave passes, the dust expands and cools back down below its original temperature.

jeffb was thinking about gas heating when it is compressed and cooling when it is rarefied. But the effect of compression is to allow more efficient cooling by radiation which pulls energy from the plasma (ionized gas). One means of radiation comes from electrons changing direction in the vicinity of other electrons. This is called free-free radiation. Bound states can also be excited by collisions with free electrons and when they radiate that removes energy from the gas. This is called bound-free rad